The present invention relates to frequency receiver circuits and is particularly applicable in the field of multi-frequency telephone dialling systems.
It is useful to be able to verify that a received signal frequency or tone is valid i.e. that it satisfies certain predetermined conditions. These conditions may relate to the tolerance on the magnitude of the frequency, to the duration of the signal at that frequency and to the length of time for which, if it is valid, the frequency remains valid. It is therefore useful to record the past history of the tone accuracy. This record can be made by sampling the frequency a plurality of times and determining the fraction of the samples which are within a particular frequency band. If the fraction is greater than a predetermined fraction, the frequency or tone can be accepted as being valid. However, such a technique alone may not give good results in the presence of high noise.